A Chromosome contains a very long DNA molecule attached to some number of proteins (to package and control DNA’s functions).
Most chromosomes are located in the nucleus of cells.
In most species all of the members have the same chromosome organization.
Function: “The behavior of chromosomes during meiosis and fertilization [accounts] for inheritance patterns” (Campbell).
The chromosome theory of inheritance
Structure of Chromatin
In the nuclei of eukaryotic cells proteins condense chromosomes into a structure called chromatin, which allows the chromosomes to fit into the nuclei. Chromatin Structure
Telomeres
A telomere is a region of repetitive DNA at the end of linear chromosmes. Function: Telomeres serve to protect the end of the chromosome from destruction.
Telomeres occur mostly in eukaryotes because most chromosomes in prokaryotic cells are circular and therefore do not have any ends to protect.
Loss of this DNA leads to cell death and a lengthening may prevent cancer cells from aging.
Abnormalities and Disorders
There any many ways in which chromosomes or their arrangement can become abnormal. Any change in the number, location, or organization of chromosomes can lead to any number of disorders.
Aneuploidy- any change in the number of chromosomes whether an increase or a decrease.
The most common example is Down syndrome which is caused by trisomy of chromosome 21. An individual with Down syndrome has three copies instead of two.
A karyotype is an image of a cell’s chromosomes. . (See examples below).
The karyotype on the left is a picture of a normal human male’s chromosomes. The image on the right is of and individual who has Down syndrome caused by and extra copy of chromosome 21.
Turner Syndrome is an example of monosomy, or a deletion of a chromosome. Women with Turner Syndrome do not have a pair of X chromosomes but instead have only one.
This leads to manly characteristics and sterility.
This condition is “the only known case in which having only 45 out of 46 chromosomes is not fatal" (Campbell).
It is most likely caused by a nondisjunction, when a pair of chromosomes does not separate.
Translocations are deformities caused by a rearrangement of parts of two unlike (nonhomologous) chromosomes.
A small number of Down syndrome cases are caused by a translocation of part of chromosome 21 onto chromosome 14.
Chronic Myelogenous Leukemia is also caused when parts of chromosomes 22 and 9 switch places.
Deletion is an abnormality in which part of a chromosome is deleted. This leads to a loss of genetic information.
Cri Du Chat (also called the “Philadelphia Chromosome”–Part of chromosome 5 is deleted. This deletion leads to mental retardation and a crying sound similar to the meow of a cat.
Recent Research
Title: Loss of Telomeric Sites in the Chromosomes of Mus Musculus Domesticus (Rodentia: Muridae) During Robertsonian Rearrangements
Researchers: Indrajit Nanda, Sonja Schneider-Rasp, Heinz Winking, and Michael Schmid
Institutions: The Department of Human Genetics and The University of Würzburg
Organisms:Mus Musculus Domesticus (House Mice)
Abstract: Mouse chromosomes possessing multiple Robertsonian rearrangements (Rb chromosomes) have been examined using fluorescencein situ hybridization with the telomeric consensus sequence (TTAGGG). No hybridization signals were detected at the primary constriction of Rb chromosomes. This observation leads us to conclude that the formation of Rb chromosomes in the mouse is invariably associated with the loss of telomeric regions.
Down Syndrome Research
Author: Len Leshin MD
Title: "Trisomy 21 The Story of Down Syndrome"
Organism: Humans
"In trisomy 21, the presence of an extra set of genes leads to overexpression of the involved genes, leading to increased production of certain products. For most genes, their overexpression has little effect due to the body's regulating mechanisms of genes and their products. But the genes that cause Down syndrome appear to be exceptions.
Which genes are involved? That's been the question researchers have asked ever since the third 21st chromosome was found. From years of research, one popular theory stated that only a small portion of the 21st chromosome actually needed to be triplicated to get the effects seen in Down syndrome; this was called the Down Syndrome Critical Region. However, this region is not one small isolated spot, but most likely several areas that are not necessarily side by side. The 21st chromosome may actually hold 200 to 250 genes (being the smallest chromosome in the body in terms of total number of genes); but it's estimated that only a small percentage of those may eventually be involved in producing the features of Down syndrome. Right now, the question of which genes do what is highly speculative. However, there are some suspects."
Genes that may have input into Down syndrome include:
Superoxide Dismutase (SOD1)-- overexpression may cause premature aging and decreased function of the immune system; its role in Senile Dementia of the Alzheimer's type or decreased cognition is still speculative
COL6A1 -- overexpression may be the cause of heart defects
ETS2 -- overexpression may be the cause of skeletal abnormalities
CAF1A -- overexpression may be detrimental to DNA synthesis
Chromosome: a structure found in the nucleus of a cell that consists of two chromatids held together by a centromere; carries DNA of cell in a single strand of DNA that is condensed into compact structure; controls functions of DNA
Chromatin: a complex of DNA and protein found in the nuclei of eukaryotic organisms, packages chromosomes
Telomeres: the tips of chromosomes that contain repetitive DNA, protecting the ends of the chromosomes from breakage and degradation; slowly shorten after multiple cell divisions; prevent chromosomes from fusing together; found in most Eukaryotes Telomerase: An enzyme which "maintains and rebuilds telomeres with each cellular division." (Pruitt) (Grey chromosomes, white telomeres)
"Each time a cell divides by mitosis, the telomeres become just a bit shorter [and] with successive cell divisions, the telomeres eventually become so short that the chromosomes are no longer able to replicate." (Pruitt)
Cancer cells contain high levels of telomerase, which causes the rapid proliferation of cancer cells.
Karyotype: visual image of the chromosome set
Abnormalities:
Deletion
Inversion
Duplication
Translocation
(1: Insertion; 2: Translocation)
(1: Deletion; 2: Duplication; 3: Inversion)
Aneuploidy: incorrect number of chromosomes (addition or deletion of chromosomes)
Examples of diseases caused by irregularities:
Down's Syndrome: "Trisomy 21" -- addition of all or part of chromosome 21
Wolf-Hirschhorn Syndrome: Deletion of short arm of chromosome 4
Klinefelter's Syndrome: Addition of X (sex) chromosome; 47 XXY (found in males)
(Left: Karyotype of male with Klinefelter's syndrome; Right: Karyotype of normal male)
Turner's Syndrome: All or part of one X (sex) chromosome missing in females (type of monosomy)
Current Research
Author: Helmstetter CE, Leonard AC, Grimwade JE.
Title: Relationships between chromosome segregation, cell shape and temperature in Escherichia coli. Abstract:
Organisms: E. coli.
Organization/Institution: Florida Institute of Technology
Abstract: The partitioning of chromosomes into daughter cells during the division of Escherichia coli is non-random. As a result, the chromosome containing the older template DNA strand has a higher probability of segregating toward the old cell pole than toward the new cell pole. The numerical value of this probability is a function of the incubation temperature. It is shown here that a recent model for explaining the physiological basis for non-random chromosome segregation also explains thetemperature dependence of the segregation process.
Bibliography:
“Chromosomes” Wikipedia. 13 Oct. 2008. Wikimedia Foundation 14 Oct. 2008.
MONICA BULLOCK Per. C
A Chromosome contains a very long DNA molecule attached to some number of proteins (to package and control DNA’s functions).Function: “The behavior of chromosomes during meiosis and fertilization [accounts] for inheritance patterns” (Campbell).
The chromosome theory of inheritance
Structure of Chromatin
In the nuclei of eukaryotic cells proteins condense chromosomes into a structure called chromatin, which allows the chromosomes to fit into the nuclei.
Chromatin Structure
Telomeres
A telomere is a region of repetitive DNA at the end of linear chromosmes.
Function: Telomeres serve to protect the end of the chromosome from destruction.
Abnormalities and Disorders
There any many ways in which chromosomes or their arrangement can become abnormal. Any change in the number, location, or organization of chromosomes can lead to any number of disorders.
Aneuploidy- any change in the number of chromosomes whether an increase or a decrease.
The most common example is Down syndrome which is caused by trisomy of chromosome 21. An individual with Down syndrome has three copies instead of two.
A karyotype is an image of a cell’s chromosomes. . (See examples below).
The karyotype on the left is a picture of a normal human male’s chromosomes. The image on the right is of and individual who has Down syndrome caused by and extra copy of chromosome 21.
Turner Syndrome is an example of monosomy, or a deletion of a chromosome. Women with Turner Syndrome do not have a pair of X chromosomes but instead have only one.
Translocations are deformities caused by a rearrangement of parts of two unlike (nonhomologous) chromosomes.
Deletion is an abnormality in which part of a chromosome is deleted. This leads to a loss of genetic information.
Recent Research
Title: Loss of Telomeric Sites in the Chromosomes of Mus Musculus Domesticus (Rodentia: Muridae) During Robertsonian Rearrangements
Researchers: Indrajit Nanda, Sonja Schneider-Rasp, Heinz Winking, and Michael Schmid
Institutions: The Department of Human Genetics and The University of Würzburg
Organisms: Mus Musculus Domesticus (House Mice)
Abstract: Mouse chromosomes possessing multiple Robertsonian rearrangements (Rb chromosomes) have been examined using fluorescencein situ hybridization with the telomeric consensus sequence (TTAGGG). No hybridization signals were detected at the primary constriction of Rb chromosomes. This observation leads us to conclude that the formation of Rb chromosomes in the mouse is invariably associated with the loss of telomeric regions.
Down Syndrome Research
Author: Len Leshin MD
Title: "Trisomy 21 The Story of Down Syndrome"
Organism: Humans
"In trisomy 21, the presence of an extra set of genes leads to overexpression of the involved genes, leading to increased production of certain products. For most genes, their overexpression has little effect due to the body's regulating mechanisms of genes and their products. But the genes that cause Down syndrome appear to be exceptions.
Which genes are involved? That's been the question researchers have asked ever since the third 21st chromosome was found. From years of research, one popular theory stated that only a small portion of the 21st chromosome actually needed to be triplicated to get the effects seen in Down syndrome; this was called the Down Syndrome Critical Region. However, this region is not one small isolated spot, but most likely several areas that are not necessarily side by side. The 21st chromosome may actually hold 200 to 250 genes (being the smallest chromosome in the body in terms of total number of genes); but it's estimated that only a small percentage of those may eventually be involved in producing the features of Down syndrome. Right now, the question of which genes do what is highly speculative. However, there are some suspects."
Genes that may have input into Down syndrome include:
MLA Works Cited
Campbell, Neil A, Jane Reece, Martha Taylor, Eric Simon. Biology Concepts and Connections.
San Francisco: Pearson Education Inc, 2006.
“Chromosomes” Wikipedia. 13 Oct. 2008. Wikimedia Foundation 14 Oct. 2008.
<http://en.wikipedia.org/wiki/Chromosomes>
Chromosomal Translocation” Wikipedia. 3 Oct. 2008. Wikimedia Foundation 14 Oct. 2008.
< http://en.wikipedia.org/wiki/Chromosomal_translocation>
Hill, Dr. Mark. “Abnormal Development - Trisomy 21 (Down Syndrome)” The University of New
South Wales. Sept. 2008. < http://embryology.med.unsw.edu.au/Defect/page21.htm>
Leshin, Len MD. "Trisomy 21: The Story of Down Syndrome" Information for Parents and Professionals. 2003.
<http://www.ds-health.com/trisomy.htm>
Nanda, Indrajit. “Loss of Telomeric Sites in the Chromosomes of Mus musculus domesticus
(Rodentia: Muridae) During Robertsonian Rearrangements.” Chromosome Research. 3
1995.
“Telomeres” Wikipedia. 10 Oct. 2008. Wikimedia Foundation 13 Oct. 2008.
<http://en.wikipedia.org/wiki/Telomeres>
Jessica Goretzke, Period G
Chromosome: a structure found in the nucleus of a cell that consists of two chromatids held together by a centromere; carries DNA of cell in a single strand of DNA that is condensed into compact structure; controls functions of DNA
Chromatin: a complex of DNA and protein found in the nuclei of eukaryotic organisms, packages chromosomes
Telomeres: the tips of chromosomes that contain repetitive DNA, protecting the ends of the chromosomes from breakage and degradation; slowly shorten after multiple cell divisions; prevent chromosomes from fusing together; found in most Eukaryotes
Telomerase: An enzyme which "maintains and rebuilds telomeres with each cellular division." (Pruitt)
- "Each time a cell divides by mitosis, the telomeres become just a bit shorter [and] with successive cell divisions, the telomeres eventually become so short that the chromosomes are no longer able to replicate." (Pruitt)
- Cancer cells contain high levels of telomerase, which causes the rapid proliferation of cancer cells.
Karyotype: visual image of the chromosome setAbnormalities:
(1: Insertion; 2: Translocation)
(1: Deletion; 2: Duplication; 3: Inversion)
Examples of diseases caused by irregularities:
(Left: Karyotype of male with Klinefelter's syndrome; Right: Karyotype of normal male)
Current Research
Bibliography:
“Chromosomes” Wikipedia. 13 Oct. 2008. Wikimedia Foundation 14 Oct. 2008.
<http://en.wikipedia.org/wiki/Chromosomes>
Helmstetter, CE, AC Leonard, and JE Grimwade. “Relationships between chromosome segregation, cell shape and temperature in Escherichia coli.”
PubMed. U.S. National Library of Medicine. 13 Oct. 2008 <http://www.ncbi.nlm.nih.gov/pubmed/1294848>.
Krogh, David. Biology: A Guide to the Natural World. 2000. N.p.: Pearson-Prentice Hall, 2004.
Pruitt, Nancy L., Larry S. Underwood, and William Surver. BioInquiry: Making Connections in Biology 2E. N.p.: John Wiley & Sons, 2003.
“Telomeres” Wikipedia. 10 Oct. 2008. Wikimedia Foundation 13 Oct. 2008.
<http://en.wikipedia.org/wiki/Telomeres>